Type 2 diabetes among children and adolescents is a newly recognized disease in the United States. Because obesity and physical inactivity are increasing in children and adolescents, the prevalence of pediatric type 2 diabetes may increase and eventually become an important cause of adult morbidity and mortality. Data on type 2 diabetes in adolescents aged 15-19 years were compared between two sources: systematic population screening of the Gila River Indian Community by the National Institutes of Health (NIH), and reported cases by the Indian Health Service (IHS) from clinics in the Southwestern US. The current NIH estimate of prevalence of type 2 diabetes was much higher than the IHS estimate (5.1% vs. 0.46%). Both data sources, however, suggest a secular increase in prevalence between 1986 and 1996-97. These data are used to discuss the principles, applications, and challenges for accurate epidemiologic assessment of type 2 diabetes in children and adolescents.